Any time this question comes up, I post this. PLEASE read this and understand - if you put an extension on a hitch NOT SPECIFICALLY DESIGNED for one, you run a very high risk of failure.
This is my true story----
I *think* I'm safe to say I haul one of the biggest/heaviest combo's on the TC forum. 2-slide Eagle Cap@~6800lbs, plus a 26' Wells Cargo 10k trailer. In my years, I've spent a lot of time and money to get my rig where it's at.
My white knuckle experience came doing exactly what you're referring to. Years ago, I had an F250, hauling an average TC at the time, and a 20' ski boat ski boat @about 4500lbs. I had a 2 foot extension on a Reese Class V hitch. Didn't think I was doing anything wrong at the time.
Went in to Torklift one day in Kent to buy some rear tie downs. They inspected my truck, said sure, and they'd even install them for free.
OK.
20 minutes later, they came and grabbed me and said they refused to work on my truck.
ok.. why?
The hitch extension had made the hitch fail. There was a large crack running down the passenger's side mount bolts to the frame.
I turned white... One good hard bump and I could've had the hitch fall right out from under the truck. I immediately had them put on a SuperHitch and haven't looked back since.
The moral of my TRUE story - DO NOT guess, estimate, or figure you can safely run a hitch extension. Only a Superhitch is engineered and DOT approved to run a hitch extension.
....I now step off my soap box.